Mucosal immune effector mechanisms-secretory antibodies and mucosal cellular responses-could enhanced immune protection against HIV by intercepting virus at mucosal surfaces and reducing this effective "challenge dose" at mucosal portals of entry. The purpose of this project is to develop immunization protocols using a live, highly-attenuated SIV vaccine candidate, that could enhance local mucosal immune responses and shorten the time required for development of immune protection against mucosal challenge. Aim 1 is to define effective mucosal immunization protocols. First, SIVmac239delta4 will be inoculated via rectum or vagina with or without a mucosal adjuvant; local immune responses will be compared to those in systemically-inoculated monkeys. Then, systemic prime/mucosal boost strategies will be tested, including live SIVmac239delta4 given IV, followed by live or inactivated SIVmac239delta4 or oligomeric envelope protein (with adjuvant) administered nasally, rectally and/or vaginally. Anti-SIV IgA, IgM and IgG antibodies in serum and secretions, and anti-SIV T cells in blood will be monitored over time. Aim II is to compare levels of SIVmac239delta4 in the mucosal after mucosal versus systemic inoculation of SIVmac239delta4, and to evaluate possible effects of attenuated SIV infection on mucosal immune function. Viral genome and antigens will be localized in rectal biopsies and correlated with the immune responses observed in Aim I. Antigen-presenting cells, IgA, IgM and IgG producing cells and T cell subsets in mucosal lymphoid follicles and lamina propria will be quantitated, and mucosal responses to test antigens evaluated. Aim III is to test the capacity of SIVmac239delta4 to protect against mucosal challenge with virulent SIV. Monkeys immunized using the optimal SIVmac239delta4 prime/boost protocol identified in Aim I will be challenged at 2 time points (in parallel with systemically-inoculated monkeys in Project III) to compare the time required for development of immune protection against mucosal challenge with that seen after systemic inoculation alone. These studies will guide future investigations of the utility of live, attenuated SIV/HIV vaccines for mucosal immune protection.